A simplified schematic view of a conventional chromatograph 100 is shown in FIG. 1. The illustrated chromatograph 100 is representative of a Hewlett-Packard 6890 Gas Chromatograph. Analytical instruments such as the gas chromatograph 100 are known for use in determining the chemical composition of a sample which is typically injected at an inlet 112 into a carrier gas stream provided by a carrier gas source 111 through a manifold 113. A fluid mixture of the sample and the carrier gas is directed through a separation column 114 located within an oven 116 and exposed to a controlled temperature environment provided by a heater 118. The separation column 114 includes a stationary phase coating on the interior of the column. The interaction of the constituent compounds in the sample with the stationary phase cause differing chemical compounds in the sample to travel through the separation column at different rates and to leave the separation column at different times. The presence of compounds in the column effluent gas is sensed by a detector 124. A detector output signal is provided to a controller 126 and a computer 122 on signal lines 128,130. The compound of interest is typically called an analyte.
A significant shortcoming in the conventional gas chromatograph is due to the loss of one or more gas streams that are typically vented to the atmosphere from the inlet 112 or the detector 124. The majority of the composition of such streams is carrier gas; for example, if the inlet 112 is constructed as a split/splitless inlet, much of the carrier gas employed by the chromatograph 100 is vented away from the inlet 112. Accordingly, a column with a 1 ml/min flow rate and a split ratio of 100:1 will vent 100 times the amount of gas actually required to carry a sample through the column 114 for an analysis. Six liters of carrier gas at inlet pressure are typically lost to the surrounding environment during one hour of analysis. However, if the carrier gas were to be conserved, such a volume of gas could easily supply a column flow for many more hours of continuous operation.
The high rate of consumption of carrier gas observed in the conventional apparatus is one of the major factors that have inhibited the development of portable instrumentation, and has also limited the deployment of most bench top (i.e., non-portable) chromatographs in underdeveloped areas of the world where cylinders of carrier gas are in short supply.
There thus exists a need for analytical instrumentation that employs a carrier gas recycling system wherein, among other factors, the flow of the carrier fluid is conserved and reused to an extent satisfactory for most analytical applications.